If you write software, you should be using Fogbugz
Not only is the software/service great, when I told them I was going to buy a copy for my personal use, they said “Just switch to the free 2 user account” Whoa.
This site, developed by the infamous “Joel on Software,” allows you to track cases (bugs, features) on a project, track time spent on those cases and predicts how long it will take to meet your goals.
This is not just for software development though. Track all of your tasks in here, use the site to track how long it really took to install that Word on a client’s machine, etc, so in 4 months you can set up a project like “Client Fred” and enter the 5 cases Fred needs completed, then let fogbugz give you a schedule spread as to how likely you’re to finish on time.
It’s really need, but you do need to read the documentation so you understand all that the software has to offer, which like Transformers, has more than meets the eye.